The present invention relates to an improved method for production of a rotor of a centrifugal compressor.
The term centrifugal compressor normally defines a machine which returns a compressible fluid at a pressure which is greater than that at which it received the fluid, by imparting to the fluid the energy necessary for the change of pressure, by means of use of one or a plurality of rotors or impellers.
Each rotor comprises a certain number of blades, which are disposed radially such as to form a certain number of passages which converge towards the centre of the rotor.
Centrifugal compressors, particularly those of the high-pressure type, are normally equipped with rotors which are tested three-dimensionally in a known manner.
More particularly, the main parts of these rotors for centrifugal compressors are the hub, the shroud and the blades.
Owing to the high density of the gases, a truly critical problem in the technical field of compressors, in particular those of the high-pressure type, is that of guaranteeing the stability of the rotor.
Combined forces, which are proportional to the density of the gas and are induced by the gas labyrinth, can cause unacceptable subsynchronous vibrations, which have detrimental effects on the said rotor.
In fact, the sensitivity of the rotor to these forces increases more or less proportionally, together with the flexibility of the rotor.
In addition, the entire dynamic performance of the rotor is particularly critical for this type of application, and the simplest means for increasing the rigidity of the rotor in the same operative conditions (i.e. with the same bearings, same temperature, same pressure of the gas, etc.), is that of using shafts which have large diameters.
The diameters both of the shafts and of the hubs have therefore been increased in the known art, such as to increase the rigidity and insensitivity to external disturbances, and thus such as to improve the characteristic rotodynamic performance of the centrifugal compressors at high pressure.
The spacers have therefore been eliminated, and the aerodynamic path is obtained directly on the shaft.
More specifically, two rings maintain the axial position: a front ring is in two pieces and a rear ring is in a single piece. In addition, a covering of material with a high level of hardness protects the shaft.
These characteristics ensure that the aerodynamic path does not change, and mean that this configuration is particularly efficient in increasing the rigidity of the shaft: however, these changes mean that a different technology must be taken into consideration for production of these rotors for centrifugal compressors.
With particular reference to the known art, it should be noted that the blades are traditionally obtained by means of milling of the hub, and subsequently welding is carried out from the interior of the opening in order to join the parts.
Another technique is used for rotors with a low flow coefficient, wherein the openings are too narrow to make it possible to carry out the welding from the interior. In this technique, the two parts are welded from the shroud.
Thus, after having carried out the welding, it is also necessary to carry out the heat treatment, which, it will be appreciated, is used to reduce the residual tensions which have formed.